The present invention relates to optoisolators and more particularly, to an optoisolator leadframe and method of manufacturing optoisolators using the leadframe.
Optoisolators are electronic components which transmit electrical information between a light source (emitter) and a light detector (receiver), without electrical connection. The light emitted can be either invisible (e.g., infrared) or can be light in the visible spectrum. An input signal to the optoisolator causes the light source to turn on. When this happens, the light detector senses the light from the source and produces an output signal proportional thereto. In this manner, the output signal follows the input signal without the need for any direct electrical connection between the input and output of the device. The input is coupled to the output optically rather than electrically; hence, the name optoisolator.
In order for an optoisolator to operate properly, the light source and light detector must be oriented properly with respect to each other. If too little light reaches the detector, then device sensitivity suffers. If too much light reaches the detector, then device speed suffers because saturation of the phototransistor on the detector reduces its switching speed. It is also important to carefully regulate the spacing between the emitter and detector. If the spacing is too great, light transfer efficiency will be decreased. If the emitter and detector are too close to one another, the isolation voltage between the input and output of the optoisolator decreases.
In the prior art, optoisolators have been manufactured by gluing a carrier containing a separate light source or plurality of light sources together with a carrier containing a light detector or plurality of detectors. After the carrier containing the light sources and detectors are permanently secured to one another, the optocoupler is tested to determine whether it meets the electrical specifications for which it was designed. In manufacturing such optoisolators, there is no precise control over the final orientation of the light source and light detector with respect to one another.
In other prior art structures, emitter and detector dies are placed side by side and a reflective dome is placed over the assembly to reflect light from the emitter to the detector. In still other known structures, overlapping strips are utilized to register an emitter over a detector. Such structures require complex molds in order to accommodate the alternating raised and recessed levels of the stacked non-coplanar strips. Additionally, tolerance variations (e.g., in the thickness of each strip) can lead to inconsistent alignment of the emitter and detector.
In manufacturing optoisolators, it is conventional to use an element known as a leadframe to form the leads through which electrical connections to the finished optoisolator can be made. A leadframe comprises a plurality of individual lead members that are tied together in a metal frame. All of the leads are electrically shorted together when they are in the leadframe. Free ends of the lead members are connected through delicate wires to proper points on the emitter and detector dies. The lead members are severed from the rest of the leadframe, and hence from one another, during a final step in the optoisolator packaging process. Once the package has been assembled and severed from the leadframe, it is too late to make any adjustment to the orientation between the light source and light detector.
It would be advantageous to provide a leadframe and a method for assembling an optoisolator that ensure proper alignment between the emitter and detector dies. It would be further advantageous if the leadframe consisted of a single strip to which both the emitter and detector dies are attached. In such an arrangement, wire bonding would advantageously occur on the same side of the same strip for both the emitter and detector dies. In order to provide proper orientation and alignment between the emitter and detector, it would be advantageous to provide a coplanar arrangement between the emitter and detector lead assemblies of the leadframe. To facilitate the manufacture of a completed optoisolator, the coplanar relationship should be maintained during initial processing when the emitter and detector dies are attached to the leadframe, and during subsequent processing when the emitter and detector dies are placed into proper orientation with respect to one another. It would also be preferable to provide means for securing the emitter and detector lead assemblies in proper alignment within the leadframe prior to encapsulation of the device into a final product.
The present invention provides a leadframe and method for assembling the leadframe into an optoisolator enjoying the advantages noted above.